Crystals of KTiOPO.sub.4 and its analogs are considered highly useful because of their nonlinear optical properties. U.S. Pat. No. 3,949,323 teaches the use of flaw-free crystals in nonlinear optical and electro-optical applications. Of the KTiOPO.sub.4 family of materials arsenate analogs (e.g., KTiOAsO.sub.4) have been recognized as having larger electro-optic coefficients, larger nonlinear susceptibility and/or broader transparency in the infrared than KTiOPO.sub.4 ; and thus can provide an improved alternative for KTiOPO.sub.4 in many applications, see, for examples, Ballman et al., Appl. Phys. Lett 54 (9), 783-785 (1989) and Cheng et al., J. of Crystal Growth, 110, 697-703 (1991).
Since KTiOPO.sub.4 and its isomorphs are known to decompose upon melting, hydrothermal and flux methods have commonly been used to grow crystals of these compounds. U.S. Pat. No. 4,305,778, as well as others teach preparation of the crystals by hydrothermal methods.
A desire for larger crystal size, better quality and greater durability, as well as the technical complexity of hydrothermal processes have led to continued interest in flux growth techniques and to the development of a variety of flux processes. In U.S. Pat. No. 4,231,838 crystal growth is carried out by heating certain mixtures of MTiOXO.sub.4 with a nonaqueous flux M/X/O (where M is selected from K, Tl, and Rb and X is selected from P and As) or their precursors to produce a nonaqueous melt. Crystal growth is affected by the use of a temperature gradient or by slow cooling of the melt at a rate of not greater than 5.degree. C./hour.
Some flux growth methods have included the use of other fluxes to improve various aspects of crystal production. The use of tungstic anhydride flux is described by Ballman, et al., "Growth of Potassium Titanyl Phosphate (KTP) from Molten Tungstate Melts" J. of Crystal Growth 75, 390-394 (1986) to improve the yield of quality crystals. The use of tungstate and molybdate fluxes for crystal growth is described in Cheng et al., "Crystal Growth of KTiOPO.sub.4 isomorphs from tungstate and molybdate fluxes" J. of Crystal Growth 110, 697-703 (1991). An improved flux process for producing KTiOPO.sub.4 and isomorphs using dopants selected from Ga, Al, and Si to lower the ionic conductivity of the resulting crystal is described in U.S. Pat. No. 5,084,206.
While reasonably large single crystals of KTiOAsO.sub.4 can be grown by the known flux processes, the resulting crystals are known to generally contain fine microscopic domains of thin (roughly about 5 to 50 .mu.m) layers of randomly oriented crystals running parallel to the natural faces (e.g., {011}) of KTiOAsO.sub.4. This random multidomain structure renders the KTiOAsO.sub.4 crystals useless for many electro-optic, piezoelectric and nonlinear optical applications. For example, Jani et al., Appl. Phys. Lett., 60 (19), 2327-2329 (1992) discloses that multidomain KTA crystals result in optical parametric oscillators with very low efficiency. G. M. Loiacono et al., Appl. Phys. Lett., 61(8), 895-897 (1992), indicates that the presence of domains in KTA causes a lack of second harmonic generation signal. There is thus a need for methods for conveniently providing potassium titanyl arsenate crystals which are useful for electro-optic, piezoelectric and nonlinear optical applications.